Polyethylene terephthalate (PET) resins for the bottle industry have traditionally been produced by solid state manufacturing methods. In solid stating, a low molecular weight PET is first made by a melt phase process prior to being pelletized and cooled. After pelletization, the PET is then passed through a crystallizer followed by a solid stating apparatus, where polymerization continues until the molecular weight (or IV) target is reached. The key feature of solid stating is that it is performed at a temperature below the melting point (Tm) of the resin so pellets are able to crystallize to very high levels. Additionally, the long annealing times in solid stating allow the crystals to become more uniform and “perfected,” which consequently increases their melting point. Typical solid-stated pellets have a Tm of about 240° C.
In contrast to solid-stated PET, the recently introduced “melt-phased only” resins are built to final IV in the primary reactor (and above Tm) without any subsequent solid phase polymerization (see for example, the following references (1) E. Van Endert, International Fiber Journal, pp 39-41, August 2006; (2) Brigitta Otto, et al, International Fiber Journal, pp 44-45, August 2006, (3) US20060046004 and (4) US20060047102. Melt phased resins have advantages in flavor sensitive products like beer, juices, water, etc. since acetaldehyde generation can be more tightly controlled (acetaldehyde is a degradation by-product of PET with a sweet, fruity flavor that forms when the resin gets too hot). For melt-phased only resins, crystallization of the pellets normally occurs from the heat retained in the material during the pelletization process so the melting point and total crystallinity are much lower than traditional solid-stated resins. If desired, additional annealing can be performed to increase the Tm and crystallinity, however, this adds an extra processing step. Typical low peak melting points (Tm1) of melt phased PET resins range from about 150 to 230° C.
Optimum melt processing conditions for traditional solid-stated polyester resins have developed over the years. These conditions include screw design for extruders and injection molding machines, operating temperatures, throughput rates or screw rpm, and the like. The optimum melt processing conditions allow maximum production rate of articles such as preforms or sheet with minimal quality defects. Given the differences in low peak melting point and crystallinity of melt-phased only resins compared to solid-stated resins, a need arises to re-optimize melt processing conditions for the new melt phased only resins.
In addition to maximizing throughput rate in terms of faster extrusion/injection rates, there is also a need for a resin and process that can be run colder than with current solid-stated PET. A colder polymer melt takes less time to solidify in the mold, and therefore can be ejected faster, giving a shorter overall cycle time. There is also a secondary benefit in that colder resins produce lower levels of acetaldehyde. The limitation on the lowest practicable melt processing temperature when processing solid-stated resins comes about because of their high Tm and very high crystallinity, preventing them from being molded below about 280 to 290° C. The melt must be made sufficiently hot so as to fully melt out all crystallinity, otherwise residual crystal defects that carry over serve as nucleation points during cooling. This nucleation significantly accelerates the rate of crystallization in the preform and can lead to unacceptable haze and poor blow molded properties in the final bottle.
Melt-phased only resins are less prone to these haze-forming residual crystals given their lower Tm and crystallinity. Therefore it is hypothesized that it should be possible to run these melt phased resins colder and with a faster cycle time. The hurdle to achieving this goal, however, has been the bubble and unmelt defects encountered when using traditional screw designs. These defects have forced processors to run hotter and/or slower in order to compensate. Thus, there is also a need for a screw design that will provide a uniform and relatively defect free extrudate at colder temperatures without sacrificing throughput rate. Such a screw would enable shorter cycle times and reduced acetaldehyde generation rates. The present invention addresses these needs.